14.6 Monitoring populations (2024)

Breeding programs should not only be evaluated for realized genetic improvement but also for the amount of inbreeding. Inbreeding takes place when related animals, that do have an additive genetic relationship higher than zero, are mated. Their offspring is inbred. And as we have seen, the inbreeding coefficient is equal to half the additive genetic relationship of the parents. Inbreeding may lead to the expression of monogenic recessive defects and to the expression of many recessive alleles with small effects measured as inbreeding depression. Inbreeding gives homozygosity at many loci and then favourable dominance effects disappear. In this respect inbreeding has the opposite effect of crossbreeding. There we have seen that crossbreeding leads to heterosis, especially exploited to improve health and fitness traits. Heterosis can be explained by thousands of partially recessive, deleterious mutations with very small effects at sites that are conserved across species.

Inbreeding depression has been widely documented for livestock and other animal and plant populations. Inbreeding is generally expected to have a stronger unfavorable effect on fitness traits than on other traits. When e.g. mating full sibs of parents with offspring in dog breeds (in The Netherlands not allowed after 1999!) a significant decrease in litter size was found. But in a large meta-analysis with different animal species inbreeding had an unfavorable effect on all sorts of traits and there was no evidence for a stronger effect on primary fitness traits (e.g., reproduction/survival traits) than on other traits (e.g., production traits or morphological traits).